The present invention relates in general to oxidation- and corrosion-resistant coatings for metals and more particularly to a process for forming an aluminide coating on the nickel- and cobalt-base superalloys.
It is known in the art to improve oxidation resistance of the various nickel-, cobalt- or iron-base alloys used in gas turbine engine applications by providing them with aluminide coatings. Typical of the coating processes used are the pack coating methods described by Wachtell et al. U.S. Pat. No. 3,257,230 and Boone et al. U.S. Pat. No. 3,544,348 and the slurry method of Joseph U.S. Pat. No. 3,102,044. These processes are utilized to form, by reaction with one or more of the substrate elements along with simultaneous and/or subsequent diffusion heat treatment, one or more different aluminides which display good oxidation-erosion resistance and thus extend the operating lifetimes of the alloy components beyond those attainable in the uncoated condition.
It is also known, as described in the U.S. Pat. Nos. to Bungardt et al 3,677,789 and 3,692,554 to apply a separate layer of metal from the platinum group before the aluminum diffusion treatment in order to increase high temperature corrosion and scale resistance. As taught by Bundgardt et al, however, the expensive platinum layer must be at lest 3 microns, preferably 7 microns, thick.